LESSON XXIX 



FERNS 



WE saw in the previous lesson that in mosses there is a 

 certain though small amount of histological differentiation, 

 some cells being modified to form sclerenchyma, others to 

 form axial bundles. We have now to consider a group of 

 plants which may be considered to be, in this respect, on 

 much the same morphological level as Polygordius, the 

 adult organism being composed not of a mere aggregate of 

 simple cells, but of various well-marked tissues. 



A fern-plant has a strong stem which in some forms, such 

 as the common Bracken (Pteris aquilina) is a horizontal 

 underground structure, and is hence often incorrectly con- 

 sidered as a root : in others it creeps over the trunks of 

 trees or over rocks : in others again, such as the tree-ferns, 

 it is vertical, and may attain a height of three or four metres. 

 From the stem are given off structures of two kinds the 

 leaves, which present an almost infinite variety of form in 

 the various species, and the numerous slender roots. In 

 some cases, such as the tree-ferns and the common Male 

 Shield-fern (Aspidium filix-mas\ the plant ends distallyin a 

 terminal bud, consisting, as in Nitella and mosses, of the 

 growing end of the stem over-arched by leaves : in others 



